Treating paper with silica sol, and product produced



',80l,938 Patented Aug. 6, 1957 TREATING' PAPER Willi-l SILICA SOL, AN DPRGDUQT PRQDUCED Ralph K. Her; Brandywine Hundred, Duh, assignor to E.I. du Pont de Nemours and Company, Wilmington, DelL, a corporation. ofDelaware NoDrawing; Application May 12, 1953, Serial No. 354,635

2 Claims. or. 117-152 This inventionrelates 'to the impregnation ofpaper withcolloidal silica and is more'particularly directed toprocesses in which fibrous, cellulosic paper is impregnated with analkali-stabilized colloidal silica solution and the. silica isthereafter irreversibly attached to the fibers of the paper, and isfurther directed to the treated paper productso produced. The manner ofattachment of the silicato the fibers of the paper is such that, afterthe paper containing the silica has been dried, the silica is notleached out by rewetting or soaking with water, and in'this sense-theattachment is said to be irreversible.

This application is a continuation-in-part of my copending United Statesapplication Serial No. 664,690, filed April 24, 1946, and now abandoned.

Various methods have hitherto been proposed for treating paper withsilicates or silica, but such methods have been characterized by usingcompositions Which contained a large proportion of sodium or otheralkali metal cations." The silicate or silica was present asprecipitated gel dispersed throughout the'paper as more or less discreteparticles of substantial size. As a consequence the finished paper had aheterogeneous composition and was hard, brittle, and tended to belacking in strength.

Attempts have also been made to treat paper with aqueous*systemscontaining silica, but all of the silicic acid compositionswhich it hashitherto been proposed to use have been products which are unstable andwhich gel upon standing. Moreover, the methods proposed for applyingsuch materials to paper have been such as tend to accelerate the gellingaction prior to thorough impregnation'of paper. As a consequence thetreated paper productproduced has resembled the product obtained byapplying gelled products derived from sodium silicate or by applyingsodium silicate and gelling it in the interstices of the paper by acidtreatment.

Now according to the present invention it has been found that paperhaving remarkably improved qualities such as increased strength,decreased brittleness and greaterresistance to scuffi'ng may be producedby processes in which a pre-formed, fibrous cellulosic paper sheet isimpregnated with an aqueous, stabilized colloidal silica solution andthe silica is irreversibly attached to the fibers of the paper.-

In describing this invention the term paper is used in its usual andordinary meaning and will be understood to include cellulosic paper inthe course of its manufacture, after the sheet has been formed and thewater content has been reduced to below about 50 percent. Also includedis paper of substantial thickness such as may commonly be referred to aspaperboard, pasteboard, cardboard, or fiberboard.

The stabilized-colloidal silica solutions which can be used according tothis invention are characterized by containing silica in the form ofpartcles having an ultimate particle size not exceeding about 130millimicrons, preferably less than 50 millimicrons, and more preferablyin the range from to 30 millimicrons. By ultimate particle size is meantthe average particle present when the solution is diluted to about .lpercent SiOz with water and dried in a very thin layer deposit.Additionally, the silica. solutions are characterized by having alkaliions'so disposed around the surfaceof the particles as to preventcondensation between the particles by oxolation in aqueous solution at aconcentration not exceeding about 45 percent SiO2 by weight.

It is possible to prepare silicic acid solutions in which the silicicacid exists as lowmolecular weight polymers of the theoreticalorthosilicic acid Si(OH)4. However, a condensationreaction, rapidlyoccurs whereby water 1 splits out betweenhydroxyl groups attached tosilicon atoms forming an Si:OSi linkage and producing a higher polymer.This process, referredto as oxolation may continue indefinitely and insuch a manner that the silicic acid solution rapidly sets up to a gel,the oxolation occurring between externally disposed hydroxyls, i. e.,hydroxyls notalready inactivated by being inwardly located within asiloxane ring or -1nicelle. It has been found, however, that it ispossible to retard such polymerization by forminga layer of alkali ionsaround the particles of such polysilicic acid while the latter is stillin an-aqueoussol.

The stabilizationof a silica sol with sodium ions may be accomplished,for instance; by procedures such as described in United. States Patent2,244,325 to Bird. However; in order to' produce stable solutions With aminimum otstabilizing agent' such as alkali, the polymerization oftheasilicicacid maybe carried to relatively high molecular weight; so asto produce large molecules or colloidal particles .whichthen retain onlya small percentage of thereactive hydroxy groups originally present, andwhich thus. require: onlya small amount of alkali as stabilizeri .On theother hand, the polymerization must "not proceedto the-formation of agel. lfsodiurn hydroxide'is used as'the alkali in aqueous solution, itleaves a negative chargeionthesilica particles so that they mutuallyrepelIeach-other and the surface hydroxyl groups which remainuon .thesurface of the particles thus have little opportunity to oxolate'andbond the particles together into still :largerunits: A

The aqueous, stabilized colloidal silica solutions-with whichpaperisstreated according to this invention are therefore."characterizedbyncontaining silica in the, form of particles'having:anultimate size ofaboutfrom 10 to millimicrons and by having alkali ions-so disposedaround the surface of the particles as,to prevent condensation betweenthe particles by oxolation in aqueous solution at a concentration notexceeding about 45 percent SiO'z by weight; The proportion of alkaliions required to stabilize a silica sol :for use in this inventiondepends upon the size of the silica particles as above-mentioned, andupon other'factors to'be described hereinafter. This proportion ofalkali can be stated conveniently as the weight ratio of total silica,expressed as SiOz, to total alkali, expressed as NaaO. The alkali,although expressed in the ratio as NazO, can be present as a base ofanother alkali metal or as a nitrogen base such as ammonia. In such acase, the Weight of the base actually present is calculated to a weightof NazO which is equivalent on the basis of alkalinity. This is a simplemolar equivalence; for example, 94 parts by weight of K20 is equivalentto 62 parts byweight of NazO. As a practical matter the alkalinitypresent is easily determined by a volumetric titration with a standardacid, and the acid used is readily calculated to NazO. Organic bases,such as morpholine, may be expressed as. NazO by calculation from such astandard acid titration.

A very practical method for producing stabilized aqueous colloidalsilica solution consists in passing a sodium silicate solution throughan acid-regenerated ion exchange resin as described in theabove-mentioned Bird patent whereby sodium ions are removed from thesodium silicate and replaced with hydrogen ions. In this instance thesodium ions may be completely removed and then sodium hydroxide may beadded to furnish the necessary sodium ions.

A particular stabilized aqueous colloidal silica solution which may beused for treating paper with especial advantage according to a processof this invention is described in the Bird patent at page 2, column 1,lines 12 to 68. As stated in the Bird patent for a silica sol producedin this manner the weight ratio of SiOz to NazO may be as high as from75 to 100:1 and as low as 10:1,

but ratios of 40:1 and 50:1 give superior results and are preferred. Thehigher the ratio, of course, the lower will be the number of sodium ionspresent. If desired, the pH of a sol prepared as described by Bird maybe adjusted downwardly to, say, 3.5 to 5.5 before applying the sol topaper.

A stabilized sol containing larger ultimate particles and henceeffective at a higher SiOzzNazO ratio can be prepared by a processcomprising building up the size of particles in an aqueous silica sol byheating to from 60 C. to about the boiling temperature at atmosphericpressure a heel consisting of an aqueous sol of silica particles, thesol, after heating, having a relative viscosity of from 1.15 to 1.60 at10 percent SiOz, adding to said heel a silica sol containing particlesof less than 10 millimicrons diameter, and continuing the addition andheating until at least 5 times as much silica has been added to the heelas was originally present, all as described in United States Patent2,574,902 to Bechtold and Snyder. Such sols have a silica:alkali ratioof from 60:1 to 130:1, and have an average particle diameter in therange from to 130 millimicrons. The sols can be highly concentrated,containing as much as 35 percent SiOz by weight, but preferably arediluted to a lower concentration before using for the impregnation ofpaper according to this invention.

By removing substantially all traces of electrolytes except thestabilizing alkali from built-up silica sols prepared as just described,sols which are stable at silica: alkali ratios of 130:1 to 500:1 can beproduced, as more fully disclosed in United States Patent 2,577,485 toI. M. Rule. Such sols are stable at concentrations up to 50 percent SiOsby weight and hence are especially advantageous to transport and handle,but again, are preferably diluted before use in impregnating paper inaccordance with the present invention. The average size of silicaparticles in the sols is in the range from 10 to 130 millimicrons.

The stabilized colloidal silica solution may be applied in either diluteor fairly concentrated form and may be applied to the paper according tomethods with which the art is already familiar in connection with theapplication of sodium silicate to paper. Thus, the sheet may be immersedin the solution or the solution may be applied from a transfer roll, byspraying, or by other mechanical applicators. The silica content of thefinished paper may be from 0.5 to percent SiOz by weight, from 0.5 to6.0 percent being preferred.

After the paper sheet has been impregnated with the colloidal silicasoution the silica may be irreversibly combined with the paper by suchmethods as have hitherto been used for precipitating silica from sodiumsilicate solutions in paper. For instance, the silica may beirreversibly combined with the paper by subjecting the treated sheet tothe action of an acid or an acidic salt, by evaporating the silicasolution to dryness, or by heating the solution to a high temperaturesuch as above about 60 C. at a concentration of SiOz in the solutionabove 45 percent.

The silica which is thus deposited in the paper is in the form ofparticles having an ultimate size less than about 130 millimicrons, andin the most preferred embodiment of the invention the ultimate particlesize is from 10 to 30 millimicrons. The particles may be cementedtogether at their points of contact with each other, but as viewed in anelectron microscope the individual identities of the particles arereadily observable, and when reference is made to ultimate size indescribing the silica in the paper products of this invention the termultimate refers to the unit particles from which such cementedstructures are built. In the paper, the ultimate particles of silica aredeposited on the individual fibers, either separately or as cementedstructures.

it has been found that when paper is treated with a colloidal silicasolution containing 6 percent SiO prepared according to the Bird patent,the ratio of SiOz to Naz-Q being about 1, and the paper is dried on acalendering roll the silica is present in the paper in an insolubleform, and is not leached out by further soaking in water. The NazO ispresent in chemical association with the silica in the original ratio of40: 1. The silica is intimately associated with the individual fibers ofthe paper. By varying the SiOzzNazO ratio of the original silicasolution applied to the paper from 10:1 to :1, the ratio of SiOz to NazOin combination in the dried paper may be similarly varied.

It will be understood that various assistants commonly used in treatingpaper may be used in conjunction with the colloidal silica solutions.Thus, pigments such as titanium oxide, fillers such as clays, whiting,and wood flour, coating agents such as casein, waxes, gums, and thelike, assistants such as wetting agents and penetrating agents andsimilar materials may be used.

The nature of this invention and its manner of application will bebetter understood by reference to the following illustrative examples.

Example I A quantity of paper having a soft, porous, Waterabsorptivestructure was treated according to this invention by immersing it in anaqueous, stabilized colloidal silica solution prepared by passing asodium silicate solution in contact with a mass of ion-exchange materialwhich had been treated with an acidic reagent. The treated paper wasdried for one hour at 110 C. The dried paper had improved strength, bothwet and dry. The character of the treated paper was such that it wasespecially well adapted for use as paper toweling, blotting paper, andthe like.

Example 2 A stabilized colloidal silica solution for use accord ing tothis invention was made up as described in the Bird Patent 2,244,325,page 2, column 1, lines 12 to 67. The sodium silicate used was acommercial grade having an SiOzzNazO weight percent ratio and a specificgravity of 425 Be, and this was diluted to 2.2 percent SiOz beforepassing over the carbonaceous zeolite. The solution was passed over thezeolite until the ratio of SiOz to NazO was approximately 50:1. Thesolution was then concentrated to 18 percent SiOz by open boiling atatmospheric pressure. In this 18 percent solution the silica was foundto be stable indefinitely. For the purposes of this test the solutionwas diluted to 2 percent SiOz immediately before using.

A quantity of unsized, bleached 30 lb./ream kraft paper was treated withthis silica solution by immersnig it for about five minutes. The paperwas then with drawn, passed through a ringer which was so adjusted thatthe amount of solution retained was equal to the dry weight of thepaper, and the paper was dried in an oven at C. for 15 minutes, The SiOzpick-up was found to be 9.1 percent by weight based on the original dryweight of paper.

The wet strength of the paper was measured by soaking the paper forabout 2 minutes, then measuring the wet strength, both in the directionof the paper-making machine and in the cross direction on a laboratorytensile strength testing machine. The results were expressed aspercentages of the strengths of the dry, untreated paper samples.

It was found that the wet strength of the treated paper was 28 percentof the original dry strength in the machine direction and 24 percent ofthe original strength in the cross direction, whereas the wet strengthof untreated controls was only 3 percent in each direction. It was alsofound that the resistance of the paper to penetration by grease wasmaterially improved.

Example 3 The unsized kraft paper of Example 2 was treated with astabilized colloidal silica solution prepared as described in Example 2except that the solution was used exactly as it came from the ionexchanger and without subsequent concentration. Thus, the silicasolution used contained 2 percent SiOz by weight.

In this solution the paper was treated by dipping for five minutes, thesaturated paper was run through squeeze rolls so adjusted that thepick-up of solution was equal to the weight of the dry paper, and thepaper thus treated was dried at 100 C. It was found that the paper hadpicked up 3.76 percent of its original weight of Si02.

On the laboratory tensile strength tester it was found that aftersoaking for two minutes in water the wet strenght in the machinedirection was 32 percent of the original dry strength and in the crossdirection was 26 percent of the original dry strength, whereas theuntreated controls had strengths of 3 to 4 percent in each direction.

It is found that paper treated as above described has substantiallyimproved surface characteristics. For instance, the resistance of suchtreated paper to scuifing is increased, the amenability of the paper toreceive surface treatment is increased, and the penetration of thesurface by non-polar materials such as petroleum oils is decreased.

The foregoing improvements are observed with various kinds of cellulosicpaper. For instance, the impregnation may be applied to such papers assized and unsized kraft, sulfite papers, paper toweling, bank-note paperand similar papers.

Iclaim:

1. In a process for treating fibrous cellulosic paper the stepscomprising impregnating the paper with an aqueous colloidal silica solcontaining combined sodium in a silica:NazO weight ratio of about from10:1 to :1 and in which the silica particles have an ultimate particlesize of about from 10 to 30 millimicrons and evaporating the sol todryness in contact with the paper, whereby the silica is irreversiblyattached to the fibers of the paper.

2. Paper having dispersed therethrough and attached to the individualfibers thereof silica particles having an ultimate particle size of from10 to 30 millimicrons, the silica being combined with sodium in anSiOzzNazO weight ratio of from 10:1 to 100:1.

References Cited in the file of this patent UNITED STATES PATENTS2,577,485 Rule Dec. 4, 1951

1.IN A PROCESS FOR TREATING FIBROUS CELLULOSIC PAPER THE STEPSCOMPRISING IMPREGNATING THE PAPER WITH AN AQUEOUS COLLOIDAL SILICA SOLCONTAINING COMBINED SODIUM IN A SILICA: NA2O WEIGHT RATIO OF ABOUT FROM10:1 TO 100:1 AND IN WHIHC THE SILICA PARTICLES HAVE AN ULTIMATEPARTICLES SIZE OF ABOUT FROM 10 TO 30 MILLIMICRONS AND EVAPORATING THESOL TO DRYNESS IN CONTACT WITH THE PAPER , WHEREBY THE SILICA ISIRREVERSIBLY ATTACHED TO THE FIBERS OF THE PAPER.